The present invention generally relates to a thin film transistor, and particularly relates to a thin film transistor including a semiconductor film made of amorphous silicon (a-Si).
Recently, for their superior characteristics as to compact size, thinness, low power consumption, light weight, etc., liquid crystal display devices have been used in variety of electric apparatuses. Particularly, active-matrix-type liquid crystal display devices adopting switching elements as active elements which offer equivalent display characteristics to those of CRT (Cathode Ray Tube) have been widely used in OA apparatuses such as personal computers, or AV apparatuses such as portable televisions, or the like. An example structure of such active-matrix-type liquid crystal devices will be explained in details in reference to FIG. 6.
FIG. 6 is a cross sectional view schematically illustrating the typical structure of the active-matrix-type liquid crystal display device. The active-matrix-type liquid crystal display device includes a TFT (Thin Film Transistor) substrate 101 and a counter substrate 102 which are connected so as to face one another, and liquid crystal 103 sealed in a space between the TFT substrate 101 and the counter substrate 102.
The TFT substrate 101 includes a transparent insulating substrate 104, and a gate electrode 105, a source bus line (not shown), a TFT (not shown) and a pixel electrode 106 connected to the TFT which are formed on the surface of the transparent insulating substrate 104 on the side of the liquid crystal 103, and further includes an alignment film 107 formed so as to cover the entire surface of the transparent insulating substrate 104 including these members. The surface of this alignment film 107 is rubbing processed.
On the other hand, the counter substrate 102 includes a transparent insulating substrate 108, and a transparent electrode 109 and an alignment film 110 which are formed in this order on a color filter (not shown) disposed on the surface of the transparent insulating substrate 108 on the side of the liquid crystal 103. The surface of this alignment film 110 is rubbing processed. In FIG. 6, reference numerals 111 and 112 indicate polarizing plates.
The structure of a conventional TFT adopted in the active-matrix-type liquid crystal display device will be explained in details in reference to FIGS. 7 and 8. FIG. 7 is a plan view illustrating a layout per pixel of the TFT substrate 101, while FIG. 8 illustrates a cross section of the portion along an arrow line Bxe2x80x94B in FIG. 8.
As illustrated in FIG. 7, the TFT substrate 101 includes gate bus lines 113 and source bus lines 114 arranged in a matrix form. Further, a gate electrode 105 and a source electrode 115 are formed as branches of the gate bus line 113 and the source bus line 114 respectively for each pixel.
Next, the concrete structure of the TFT will be explained while explaining the manufacturing steps thereof in reference to FIG. 8.
First, the gate electrode 105 is formed on the transparent insulating substrate 104, and thereafter, a gate insulating film 116 is formed thereon so as to cover the gate electrode 105. Then, on this gate insulating film 116, an amorphous silicon semiconductor layer 117 without impurity and an amorphous silicon semiconductor layer 118 with impurity are patterned in a shape of island. Further, the source electrode 115 and the drain electrode 119 are formed by etching by setting an etching selectivity ratio to the amorphous silicon semiconductor layer 118 with impurity (the amorphous silicon semiconductor layer 118 is not etched completely). Then, this amorphous silicon semiconductor layer 118 with impurity is subjected to etching to form a source/drain separated portion, and further the pixel electrode 106 is formed by the transparent electrode. Thereafter, the entire surface of the TFT substrate 101 is covered with an protective film 120.
Hereinafter, a path for flowing therethrough an OFF-state current (to be described later) formed on the surface or the interface of the amorphous silicon semiconductor layer 117 without impurity at a portion (source/drain separated portion) between the source electrode 115 and the drain electrode 119 is referred to as a back channel.
However, for the thin film transistor manufactured by the foregoing conventional method, the following problem remains unsolved. That is, in the state an electric field induced by the externally applied positive charges due to the surface contaminations or by the positive charges of the protective film itself becomes not less than the threshold level of the back channel, the OFF-state current value of the TFT (leak current value in the OFF state) increases due to the back channel effects. The ratio of the ON-state current to the OFF-state current of the TFT determines the contrast of the display device which greatly affects the display quality. Further, the described phenomenon of increasing the OFF-state current value of the TFT due to the back channel effects is induced by driving the thin film transistor over a long period of time, and the foregoing problem of an increase in the OFF-state current is serious as it affects the reliability of the product.
Here, the back channel effects are defined to be a phenomenon in which electrons are induced in the back channel by the externally applied positive charges due to the surface contaminations or by the positive charges of the protective film itself.
Here, as a solution to the above problem, for example, Japanese Unexamined Patent Publication No. 8440/1996 (Tokukaihei 8-8440 published on Jan. 12, 1996) discloses a structure wherein a p-type amorphous silicon layer is formed between the amorphous silicon semiconductor layer and the protective film in the back channel region so as to prevent an increase in OFF-state current value caused by electrons induced in the back channel.
However, according to the structure of the thin film transistor disclosed in Japanese Unexamined Patent Publication No. 8440/1996, a problem arises in that for the formation of this p-type amorphous silicon layer, the required number of steps for manufacturing the thin film transistor increases, resulting in an increase in manufacturing cost.
According to the foregoing conventional method, it is therefore not possible to provide the thin film transistor which realizes a further reduction in OFF-state current and which can be manufactured at lower cost through a smaller number of steps.
It is an object of the present invention to provide a thin film transistor which realizes a further reduction in OFF-state current and which can be manufactured at lower cost through a smaller number of steps.
In order to attain the foregoing object, a thin film transistor of the present invention is arranged so as to include:
a gate electrode provided on a transparent insulating substrate;
a first semiconductor layer formed on the gate electrode via a gate insulating film; and
a source electrode and a drain electrode formed on the first semiconductor layer via a second semiconductor layer which functions as a contact layer,
wherein protrusions and recessions are formed in a separated portion between the source electrode and the drain electrode on a surface of the first semiconductor layer.
Here, the surface of the first semiconductor layer between the source electrode and the drain electrode is referred to as a back channel region (a region where a back channel is formed).
In the foregoing structure, the non-bonded area where bonds are cut is increased in the back channel region by the surface protrusions and recessions formed in the source/drain separated portion on the surface (back channel region) of the first semiconductor layer, and the number of uncombined bonds increases consequently. Accordingly, defects which trap carriers increase in the back channel region, and the effect of bending a band can be suppressed, and whereby a threshold value of the back channel can be increased. In the state where an electric field induced by the externally applied positive charges due to the surface contaminations or by the positive charges of the protective film itself becomes not less than the threshold level, the OFF-state current value of the TFT increases. It is therefore possible to reduce an OFF-state current value by increasing the threshold value of the back channel as in the foregoing structure of the present invention.
Furthermore, by suppressing an increase in OFF-state current value, such problem that the final product becomes less reliable due to a reduction in contrast over a long time driving of a panel can be more surely prevented.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.